1/3 cup coarsely chopped fresh cilantro2 medium garlic cloves, finely chopped 1/2 teaspoon chili powder 1/2 teaspoon ground coriander 1/2 teaspoon ground cumin Juice of 2 limes 3 tablespoons olive or vegetable oil 1 pound boneless, skinless chicken breast Kosher salt Freshly ground black pepper 8 (6-inch) tortillas (corn or flour) 3 to 5 sweet peppers (any color), cored and sliced into 1/2-inch strips 1 medium onion, halved and sliced into 1/2-inch pieces Guacamole for serving (optional) Salsa for serving (optional) Sour cream, for serving (optional)
Place the cilantro, garlic, chili powder, coriander, cumin, lime juice, and 2 tablespoons of the oil in a shallow baking dish and whisk to combine. Add the chicken and turn to coat with the marinade. Set the chicken aside for at least 10 minutes, or cover and refrigerate for up to 24 hours.
Heat a grill pan to medium. Once hot, add the chicken pieces, season with salt and pepper, and cook undisturbed until well browned on the bottom, about 10 minutes. Flip, season the second side with salt and pepper, and cook undisturbed until well browned on the bottom and cooked through, about 10 minutes more. Remove the chicken to a cutting board and let it rest while you prepare the remaining ingredients. Set the grill pan aside.
Warm the tortillas by heating a medium cast iron or frying pan over high heat until hot, about 3 minutes. Place a tortilla in the dry pan and heat, flipping once, until warmed on both sides. Wrap the tortilla in a clean dishtowel to keep warm. Repeat with the remaining tortillas.
Place the bell pepper and onion in a medium bowl, drizzle with the remaining 1 tablespoon oil, season with salt and pepper, and toss to coat. Return the grill pan to medium heat and add the vegetables in a single layer. Cook, stirring occasionally, until tender and slightly charred, about 10 minutes. Remove the vegetables to a serving dish.
Slice the chicken into 1/2-inch-thick pieces and place in the serving dish with the vegetables. To serve, fill a warm tortilla with chicken and vegetables and top with guacamole, salsa, and sour cream (if using).